Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. In operation, a typical contact start/stop (CSS) method commences when a data transducing head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk where it is maintained during reading and recording operations. Upon terminating operation of the disk drive, the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparent competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by techniques generally referred to as "texturing." Conventional texturing techniques involve mechanical polishing or laser texturing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers.
A typical longitudinal recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Al--Mg)-alloy, plated with a layer of amorphous nickel-phosphorus (NiP). Alternative substrates include glass, glass-ceramic materials and graphite. Substrate 10 typically contains sequentially deposited on each side thereof a chromium (Cr) or Cr-alloy underlayer 11, 11', a cobalt (Co)-base alloy magnetic layer 12, 12', a protective overcoat 13, 13', typically containing carbon, and a lubricant topcoat 14, 14'. Cr underlayer 11, 11' can be applied as a composite comprising a plurality of sub-underlayers 11A, 11A'. Cr underlayer 11, 11', Co-base alloy magnetic layer 12, 12' and protective overcoat 13, 13', typically containing carbon, are usually deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface to provide a texture, which is substantially reproduced on the disk surface.
In accordance with conventional practices, a lubricant topcoat is uniformly applied over the protective layer to prevent wear between the disk and head interface during drive operation. Excessive wear of the protective overcoat, typically comprising carbon, increases friction between the head and disk, thereby causing catastrophic drive failure. Excess lubricant at the head-disk interface causes high stiction between the head and disk. If stiction is excessive, the drive cannot start and catastrophic failure occurs. Accordingly, the lubricant thickness must be optimized for stiction and friction.
Thus, a significant factor in the performance of recording media is the thickness of the topcoat lubricant. The lubricant topcoat is typically applied at a thickness no greater than necessary to maintain the requisite lubricity.
It is desirable to employ lubricants which form an effective functional topcoat at a thickness less than those conventionally employed for improved medium performance. Perfluoropolyether lubricants with one or more functionalized end-groups are conventionally employed for recording media topcoats. The functionalized end-groups of these compounds are believed necessary to improve adhesion of the lubricant topcoat to the recording media. It is also believed, however, that for functionalized perfluoropolyether lubricants to provide the requisite tribiology, they must be applied at a relatively high topcoat thickness, particularly when the recording medium is expected to perform under high stress conditions.
Gui et al., in U.S. Pat. No. 5,562,965, disclose the use of terminally functionalized perfluoropolyether coatings of thin film disks. Gui et al. further teach that the functionalized perfluoropolyether lubricants have a molecular weight not more than 4 kD, preferably within the 2-3 kD range. It is also known that alcohol derivatized perfluoropolyether lubricants exhibit both high stiction and a rapid increase in dynamic friction or wear under conditions of high temperature and humidity.
Perfluoropolyether lubricants with nonfunctionalized end-groups are also known. However, such lubricants have not found significant use as disk lubricants since they suffer from poor wear resistance. Moreover, the components of commercially available terminally nonfunctional perfluoropolyether lubricants typically have wide and varying distributions of molecular weight.
In view of the criticality of the lubricant topcoat, there is a continuing need for improved lubricant stiction and wear performance, particularly under conditions of high temperature and humidity. There is also a need for lubricants which form functionally effective topcoats on recording media at a thickness less than conventional lubricant topcoats.